Phase control method of stopping a drive smoothly

ABSTRACT

A method of smoothly stopping pump drives without pressure surges is provided. To accomplish this, the phase control angle between the motor current and the motor voltage is increased according to a linear ramp starting from 0° with the help of an a.c. power controller, where the phase angle decreases and reaches a turning point. If the turning point is detected by observing the phase angle, the phase control is based on this by means of a controller with a predefined linear phase angle reference curve as a function of time. Then the motor rpm does not change abruptly because a rapid change in phase angle is prevented.

FIELD OF THE INVENTION

The present invention relates to a phase control method of stopping adrive smoothly, in particular a pump drive, in accordance with the phaseangle between the voltage and the current of the drive motor.

BACKGROUND INFORMATION

There are known phase control methods of stopping and starting a drivesmoothly as a function of the phase angle between the motor voltage andthe motor current. Smooth stopping makes special demands on the controlsystem. When a pump drive is shut off, it causes knocking in pipelinesystems which is also known as "water hammering" and is caused by theabrupt changes in flow. This not only damages the pipeline system but isalso disturbing because of the acoustic noise.

An abrupt change in flow occurs especially when the drive of a rotarypump, for example, is shut down directly by opening the respective motorcontactor. The following relationship holds between the rpm and flow andbetween the change in rpm and the change in pressure: the flow isdirectly proportional to pump rpm, and the change in pressure isdirectly proportional to the square of the change in rpm.

As FIG. 1 illustrates, in the case of a direct shutdown of a pump drivewith a three-phase induction motor, the drive is stopped within a veryshort period of time because of the load torque curve Pum in which thepump has a high counter-torque. FIG. 1 shows the moment M plottedagainst the rpm n. Mom is the motor torque, Pum is the load torque, Bmis the accelerating torque, Kim is the breakdown torque and N is thenominal working point. Because of the high counter-torque Pum of thepump, there is a rapid change in rpm n, resulting in an abrupt change inflow and pressure because of the above-mentioned relationships. Thisleads to a pressure surge known as a water hammer. Any check valves willexacerbate the problem.

FIG. 2 shows a plot of motor current I, motor voltage u and rpm n overtime in a direct disconnect.

In the past, various methods have been used as countermeasures.

Mounting an inertial weight on the pump drive makes it possible toprevent the drive from stopping immediately when there is a shutdown.The stored energy of the inertial weight leads to delayed stopping ofthe pump, so the change in pressure and the change in flow are sloweddown. This method is complex mechanically, and additional power isneeded for operation.

By using frequency converters, the pump motor can decelerate from itsrated speed N according to an rpm ramp. This method is comparativelyexpensive, especially for higher-powered pumps.

So-called smooth starting devices that operate according to theprinciple of a 3-phase a.c. power controller are used for turning pumpmotors on and off. In addition to smooth starting, these devices alsopermit smooth stopping. Through phase control, the motor terminalvoltage is not disconnected suddenly but instead is ramped down, so themotor also stops smoothly.

There are different types of smooth stopping. The simplest type consistsof increasing the phase control angle linearly from 0° until the motorhas stopped. As FIG. 3 illustrates, if the reduced motor torque dropsbelow the load torque, there is a rapid change in rpm, which in turncauses water hammering in most cases. Another method consists ofmeasuring the motor terminal voltage and ramping it down when stopping.However, additional hardware is needed to measure the motor terminalvoltage. One problem here is that the voltage is phase-controlled andfurthermore it must be supplied in a potential-free form for control. Ina simple solution a time lag that can have a negative effect on thecontrol response occurs due to the measurement and "smoothing" of themotor terminal voltage.

The present phase control method of smoothly stopping is based on theprinciple of a known smooth starting device according to FIG. 4, as isdescribed in German Published Patent Application 4,005,679. A 3-phaseinduction motor Mt is connected to the three phases A, B and C of a3-phase system via a thyristor circuit of anti-parallel thyristors 1A,1B and 1C, respectively. The six thyristors can be energized by means ofa suitable ignition circuit. If the thyristors are "on" after ignition,the current flow stops when the current IA, IB, and IC passes throughzero. Phase control is defined as occurring when the thyristors areenergized again a predefined period of time after the current stops ina.c. applications. The delay is usually known as the phase angle.

In addition to the above-mentioned circuit of the a.c. power controllerand the ignition circuits, the basic design of the smooth startingdevice also includes detection circuits 3A, 3B, and 3C that display thestatus of a pair of thyristors. Only the two states "current carrying"and "de-energized" are differentiated, as indicated by the curve IOA, inFIG. 5.

A detection circuit 4 that supplies a line-synchronous signal from theline voltage VAB serves as a synchronizing signal (VOAB). This signaldifferentiates only between the two states "positive voltage" and"negative voltage" and is derived from the voltage VAB between the twophases A and B according to FIG. 5.

With the help of these signals it is possible to determine the phaseangle PA between the motor current and the voltage.

The phase angle PA between the motor voltage and the motor current isdetermined from a signal such as IOA, which indicates the status of theindividual thyristor pairs 1A, 1B and 1C. Specifically, signal IOAindicates the time at which no current passes through thyristor pairs1A, 1B and 1C. This status is indicated by the trailing edges in thecurve IOA, for example, and from the signal VOAB, which is synchronouswith the line voltage VAB and indicates in particular when the linevoltage is zero. FIG. 5 shows these relationships including the ignitionangle control according to the curve labeled as FPA for thyristor pair1A in phase A.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of smoothlystopping pump drives without causing pressure surges, so the pipelinesystem is not damaged and negative acoustic effects are also largelyprevented. This is accomplished with a phase control method of smoothlystopping a drive, specifically a pump drive, in accordance with thephase angle between the voltage and the current of the drive motor. Inaccordance with this method, the change in phase angle over time is atfirst negative due to the increase in the phase control angle. The phaseangle changes directions at a turning point. Further, starting from theinstantaneous phase angle, the phase control angle is regulated using apreset phase-angle reference curve as the input parameter when theturning point is determined with the help of a controller. In thismethod, no additional hardware is needed in comparison with theconventional control for smooth starting devices with phase control.Only the phase angle between the current and the voltage is needed forthis method, and this value is either already available by conventionalcontrol methods or it can be easily derived from the availableinformation.

It is especially advantageous if the phase control angle increaseslinearly until the turning point is detected. Furthermore, it isadvantageous if the phase angle reference curve is linear with apositive change in phase angle over time. The increase in phase controlangle is preferably in the range of 15° to 20° per second. It is alsoexpedient if the linear increase in the phase angle reference curve overtime is in the range of 0.5° to 6° per second. An especiallyadvantageous embodiment of this method is obtained if the phase controlangle is controlled in the following steps with the predefined linearphase angle reference curve for the positive change in phase angle overtime:

a) the measured phase angle is compared with the phase angle referenceat equidistant time increments Δt;

b) the instantaneous phase angle is sent to the controller as thestarting value for the comparison value when the turning point isdetected;

c) if the phase angle measured after the time increment Δt is greaterthan the reference at that time, the phase control angle is reduced by afirst differential value in comparison with the latest value; otherwisethe first differential value is added to the latest value;

d) the comparison value is increased by a second differential valueaccording to the reference at the next time increment Δt;

e) the steps described above are repeated with the phase angle measuredat the next time increment Δt and the corresponding comparison value, aslong as the resulting new phase control angle is smaller than thelargest possible phase control angle.

The slope of the linear phase angle reference curve is preferablyadjustable in order to be able to influence the stopping time.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the following detaileddescription, when taken in conjunction with the accompanying drawings,in which:

FIG. 1 shows the moment M plotted against the rpm n;

FIG. 2 shows motor current I, motor voltage u, and rpm n plotted againsttime;

FIG. 3 shows motor current I, motor voltage u, and rpm n plotted againsttime when the motor torque drops below the load torque;

FIG. 4 illustrates a conventional smooth starting device;

FIG. 5 illustrates the variation over time of various signals in thedevice of FIG. 4;

FIG. 6 shows a diagram of the phase control angle and the phase angleover time;

FIG. 7 shows a phase control flow chart;

FIG. 8 shows a diagram for stopping a pump by the method according tothis invention.

FIG. 6 shows the phase control method according to this invention forstopping a pump drive smoothly. When the phase control angle D iscontrolled according to the lower curve in the diagram, this yields thephase angle PA between the voltage and the current of the drive motoraccording to the top curve. To stop the pump motor, beginning first witha phase control angle of D=0°, the angle is increased according to alinear ramp, leading to a drop in motor voltage, and the change in phaseangle over time, dPA/dt, is negative at first. The phase angle PA isdetermined in each system period, i.e., at equidistant time incrementsΔt, for example, according to the prior art method described above. Thecurve for the phase angle PA is plotted during the linear increase inthe phase control angle D and evaluated. With an appropriate choice ofthe slope for the linear rise in phase control angle D in the range of15° to 20° per second, a turning point W is obtained in the curve ofphase angle PA. If such a turning point W is detected, this is anindication that the motor is in a state where the pump torque (loadtorque) exceeds the motor torque if there is a further reduction involtage, and there is an abrupt drop in rpm which results in waterhammering. After this point in time t1, phase control angle D no longerhas a linear rise, but instead is influenced by a controller so thatphase angle PA increases according to a linear ramp, preferably afterthe turning point W. FIG. 6 illustrates phase angle PA as it increasesaccording to a phase angle reference curve R(t). As a result, the motorrpm does not change abruptly because the motor voltage is regulated viathe phase control angle D. Consequently, a rapid change in phase angleis avoided. As FIG. 6 shows, the measured phase angle PA does not followthe phase angle reference curve R(t) in the present embodiment forphysical reasons. In the initial phase after turning point W, phasecontrol angle D is almost constant at first. In other words, the motorvoltage is also kept constant, and only later is the voltage reduced byincreasing phase control angle D.

A PID controller, for example, whose controlled variable is phase anglePA is used as the controller. The manipulated variable is the phasecontrol angle D for the phase control. The reference value of thecontrolled variable increases according to the linear ramp R(t) startingfrom the instantaneous value of the phase angle PA_(w) on detection ofturning point W up to a maximum value. The slope of the linear ramp R(t)is preferably adjustable over the parameter "stopping time."

FIG. 7 shows a flow chart for the phase control according to thisinvention. According to this flow chart, phase control angle D increaseslinearly, starting from 0°, which leads to a decrease in the phase anglePA. In other words, it results in a negative change in phase over time,dPA/dt. After each time increment Δt, in other words, in each systemperiod in the present case, phase angle PA is measured as describedinitially. A first differential value of the change in phase control dDis calculated for the linear ramp so that the slope is neither toosteep, because otherwise the motor breakover point might be skipped, noris it too small, because otherwise the turning point W could not bedetected. If dPA/dt, the change in phase angle over time, is no longernegative, this is an indication that the turning point W has beenreached. The corresponding phase angle PA, is the initial reference orthe comparison value R for the controller. Starting at this time, thephase control is handled by the controller, which receives the linearphase angle reference curve R(t) as a reference or comparison value. Inother words, the phase angle comparison value R is increased in a loopby the change in comparison value dR, which is calculated from theselected stopping time. The change in comparison value dR is obtainedfrom the coordinates PA_(w) and t₁ for the turning point W and thecondition that the phase angle should be 120° at the time t₂ =t₁ +Tafter the selected stopping time T. Consequently, the change incomparison value dR within one time increment At is calculated as dR=(120°-PA_(w))/T!·Δt. Within the loop there is an inquiry as to whetheror not the measured phase angle PA is greater than the comparison valueR. For the case when PA>R, phase control angle D is reduced by adifferential value dD1 in comparison with the latest value. In theembodiment shown, the change dD1 is calculated with a PID controller.However, if phase angle PA is smaller than or equal to the reference R,there is a corresponding increase in phase control angle D by thedifferential value dD1. Subsequently, the reference R for the phaseangle is increased by the change in the comparison value dR and thus thenext control step is initiated again until as the largest possiblecontrol angle Dmax is achieved. This ends the smooth stopping for thepump drive.

FIG. 8 shows the motor current I, motor voltage u, and motor rpm n overtime for smooth stopping of a pump drive by the phase control methodaccording to this invention. An rpm curve without any abrupt dips isachieved by a controlled voltage drop. The rpm n at first drops slowlybut continuously, developing into a linear range with a steeper slope.The slope of this linear descending range depends on the preset slope ofthe linear phase angle reference curve R(t) and can be adjusted throughthe "stopping time."

What is claimed is:
 1. A phase control method for smoothly stopping apump drive motor in accordance with a phase angle between a voltage anda current of the pump drive motor, comprising the steps of:a)determining a change in the phase angle over time, the change in thephase angle at first decreasing in magnitude over time, the phase angledecreasing in magnitude due to an increase in a phase control angle; b)detecting a turning point at which the change in the phase angle beginsto increase in magnitude over time; c) determining an instantaneousphase angle at the turning point; and d) regulating the phase controlangle when the turning point is detected, wherein the regulating stepoccurs under a control of a controller having a preset phase anglereference curve as an input parameter.
 2. The phase control methodaccording to claim 1, wherein the phase control angle increases linearlyuntil the turning point is detected.
 3. The phase control methodaccording to claim 1, wherein the phase angle reference curve is linear,the phase angle reference curve increasing in magnitude over time. 4.The phase control method according to claim 2, wherein the linearincrease in the phase control angle is in a range of 15° to 20° persecond.
 5. The phase control method according to claim 3, wherein thelinear increase in the phase angle reference curve over time is in arange of 0.5° to 6° per second.
 6. The phase control method according toclaim 1, wherein the step of regulating the phase control anglecomprises:e) comparing the phase angle with the phase angle referencecurve at a plurality of equidistant time increments, the phase anglereference curve being initialized to a comparison value equal to theinstantaneous phase angle when the turning point is detected; f)reducing the phase control angle by a first differential value when thephase angle is greater than the phase angle reference curve; g)increasing the phase control angle by the first differential value whenthe phase angle is no more than the phase angle reference curve; h)increasing the comparison value by a second differential value at a nexttime increment in accordance with the phase angle reference curve; andI) repeating steps e) through h) while the phase control angle issmaller than a predetermined maximum value.
 7. The phase control methodaccording to claim 3, wherein a slope of the linear phase anglereference curve is adjustable.